HVAC actuators such as spring return actuators are used in a wide variety of applications, including but not limited to air dampers, water valves and the like. Typically, a spring return actuator has a motor that drives a drive train. The drive train often functions as a sort of transmission, turning a low torque, high speed motor output into a high torque, low speed drive train output that is sufficient to open an air damper, a water valve, or the like. A spring return actuator may have one or more return springs that oppose a driving direction of the motor. For example, a spring return actuator may be configured such that the motor drives an HVAC component (e.g. damper) from a closed position to an open position, while the return spring drives the HVAC component from the open position to the closed position. In other cases, a spring return actuator may be configured such that the motor drives an HVAC component from an open position to a closed position, while the return spring drives the HVAC component from the closed position to the open position.
In many spring return actuators, the motor produces a constant maximum output torque while the opposing force produced by the return spring varies in accordance with the relative displacement of the spring as the actuator moves between open and closed positions. The constant maximum output torque of the motor is often set to meet a minimum torque rating (e.g. 5 N·m, 10 N·m, etc.) at the HVAC component across the full range of motion of the actuator.
In some cases, a spring return actuator may have one or more end stops, either internally within the spring return actuator itself or perhaps in conjunction with the HVAC component that is driven by the spring return actuator. When the motor moves the spring return actuator and impacts an end stop, force is transferred through the motor, drive train and/or the HVAC component. Because the torque of the motor is often set to meet a minimum torque rating (e.g. 5 nm, 10 nm, etc.) at the HVAC component across the full range of motion of the actuator, the torque produced by the motor and thus the force that is transferred through the motor, drive train and/or the HVAC component when an end stop is impacted can be significantly higher than the minimum torque rating of the actuator. This is particularly so when an end stop is reached when the return spring is not fully displaced, i.e. not providing a maximum opposing force to the motor. As such, the drive train and related components often must be built to withstand substantially higher torques than the minimum torque rating of the actuator. This, however, can significantly increase the cost of such spring return actuators.